Since this had never been "officially" established, I split the difference and made him Khan's foster son in the book.

And it works rather well. Some people guess that Joachim in ST II is simply an Aryan-revised, misspelled version of "Space Seed"'s dark-haired Joaquin (Mark Tobin).

Judson Scott is one of these people, and has been since the '80s. He even insists that Joachim, which Ricardo Montalban pronounced onscreen as "YO-uh-KEEM", is pronounced "wah-KEEM". Or at least he did when he was at the Denver Star Trek convention last.